A cell permeable inhibitor of the PI3K catalytic subunit p110δ (IC50 = 2.5 nM) that demonstrates 40- to 300-fold selectivity against other PI3K class I enzymes (IC50s = 820, 565, and 89 nM for p110α, β, and γ, respectively); blocks constitutive PI3K signaling in malignant B-cell lines and primary patient tumor cells, resulting in decreased phosphorylation of Akt and other downstream effectors, an increase in poly(ADP-ribose) polymerase and apoptosis.